Base Class: Barbarian
You gain resistances to the things your ancestral parent is, in addition you have a vulnerability to radiant, and can sprout wings in the form of demonic bats
Fiendish Resistances
While raging, you gain the resistances to the following: cold, fire, lightning, and necrotic, as well as piercing, slashing, and bludgeoning from non-magical weapons. You do gain a vulnerability to Radiant as a part of this path.
Fiendish Appeal
You gain proficiency with the Persuasion and Deception skills. You also have the ability to sprout bat like wings from your back and you gain a flight speed equal to your movement speed. You cannot fly while wearing heavy armor, and unless your clothes or armor is made to accommodate the wings growth from your back they can be destroyed. You cannot fly while carrying another person as well. You also cannot be charmed or frightened while raging.
Fiendish Vision
You gain 120ft of Devil's sight, in which you can see normally in darkness, regardless if it's magical or normal darkness.
Rage Beyond Death
While you're raging, having 0 hit points doesn’t knock you unconscious. You still must make death saving throws, and you suffer the normal effects of taking damage while at 0 hit points. However, if you would die due to failing death saving throws, you don’t die until your rage ends, and you die then only if you still have 0 hit points.